Mice bearing the autosomal recessive mutation 1pr were studied for abnormalities of hematopoietic cells related to the autoimmune disease produced by this gene. C3H mice bearing this mutation were found to have marked expansion of an abnormal population of cells with surface characteristics of both T cells and B cells. As immunoglobulin heavy-chain genes were not reorganized in these cells, the 1pr gene results in a population of aberrant T cells that express B lineage antigens. SJL mice homozygous for 1pr die of severe autoimmune disease and express markedly increased levels of infectious ecotropic murine leukemia viruses whereas heterozygous 1pr mice express lower levels of virus and die with accelerated B cell tumors. However, continuous in vitro cell lines established for spleens and lymph nodes of homozygous or heterozygous mice have yielded granulocytic tumors.